Sew the cut. Using a needle and tread, the cut can be sown from the inside. This makes the shirt look like it has a small crease. There are magic stitching items for individuals who do not know how to sew. These can be used by applying the stitching, and ironing it onto the clothing.
Apply a patch. A patch can be the same color as the fabric. The T-shirt can be cut in a square or rectangular shape to fit the patch. The patch is then stitched over the T-shirt. Consider matching patches in more than one place for a cohesive look.
Cut multiple areas to match. The cut may just be a fashion statement. Making the T-shirt look like the cut was meant to be that way may prove to be a genius fix. Place similar cuts over the shirt.